2,027 research outputs found
Shear induced breaking of large internal solitary waves
The stability properties of 24 experimentally generated internal solitary waves (ISWs) of extremely large amplitude, all with minimum Richardson number less than 1/4, are investigated. The study is supplemented by fully nonlinear calculations in a three-layer fluid. The waves move along a linearly stratified pycnocline (depth h2) sandwiched between a thin upper layer (depth h1) and a deep lower layer (depth h3), both homogeneous. In particular, the wave-induced velocity profile through the pycnocline is measured by particle image velocimetry (PIV) and obtained in computation. Breaking ISWs were found to have amplitudes (a1) in the range a1>2.24 āh1h2(1+h2/h1), while stable waves were on or below this limit. Breaking ISWs were investigated for 0.27 0.86 and stable waves for Lx/Ī» < 0.86. The results show a sort of threshold-like behaviour in terms of Lx/Ī». The results demonstrate that the breaking threshold of Lx/Ī» = 0.86 was sharper than one based on a minimum Richardson number and reveal that the Richardson number was found to become almost antisymmetric across relatively thick pycnoclines, with the minimum occurring towards the top part of the pycnoclinePostprintPeer reviewe
Isolated Star Formation: A Compact HII Region in the Virgo Cluster
We report on the discovery of an isolated, compact HII region in the Virgo
cluster. The object is located in the diffuse outer halo of NGC 4388, or could
possibly be in intracluster space. Star formation can thus take place far
outside the main star forming regions of galaxies. This object is powered by a
small starburst with an estimated mass of \sim 400\msun and age of \sim
3\myr. From a total sample of 17 HII region candidates, the present rate of
isolated star formation estimated in our Virgo field is small, \sim 10^{-6}
Msun arcmin}^{-2} yr^{-1}. However, this mode of star formation might have
been more important at higher redshifts and be responsible for a fraction of
the observed intracluster stars and total cluster metal production. This object
is relevant also for distance determinations with the planetary nebula
luminosity function from emission line surveys, for high-velocity clouds and
the in situ origin of B stars in the Galactic halo, and for local enrichment of
the intracluster gas by Type II supernovae.Comment: 5 pages, LaTeX, 1 figure. ApJ Letters, in press (scheduled Dec 1,
2002
Using affective avatars and rich multimedia content for education of children with autism
Autism is a communication disorder that mandates early and
continuous educational interventions on various levels like the everyday social, communication and reasoning skills. Computer-aided education has recently been considered as a likely intervention method for such cases, and therefore different systems have been proposed and developed worldwide. In more recent years, affective computing applications for the aforementioned interventions have also been proposed to shed light on this problem.
In this paper, we examine the technological and educational needs of affective interventions for autistic persons. Enabling affective technologies are visited and a number of possible exploitation scenarios are illustrated. Emphasis is placed in covering the continuous and long term needs of autistic persons by unobtrusive and ubiquitous technologies with the engagement of an affective speaking avatar. A personalised prototype system facilitating these scenarios is described. In addition the feedback from educators for autistic persons is provided for the system in terms of its
usefulness, efficiency and the envisaged reaction of the autistic persons, collected by means of an anonymous questionnaire. Results illustrate the clear potential of this effort in facilitating a very promising autism intervention
Ares I Scale Model Acoustic Tests Instrumentation for Acoustic and Pressure Measurements
The Ares I Scale Model Acoustic Test (ASMAT) was a development test performed at the Marshall Space Flight Center (MSFC) East Test Area (ETA) Test Stand 116. The test article included a 5% scale Ares I vehicle model and tower mounted on the Mobile Launcher. Acoustic and pressure data were measured by approximately 200 instruments located throughout the test article. There were four primary ASMAT instrument suites: ignition overpressure (IOP), lift-off acoustics (LOA), ground acoustics (GA), and spatial correlation (SC). Each instrumentation suite incorporated different sensor models which were selected based upon measurement requirements. These requirements included the type of measurement, exposure to the environment, instrumentation check-outs and data acquisition. The sensors were attached to the test article using different mounts and brackets dependent upon the location of the sensor. This presentation addresses the observed effect of the sensors and mounts on the acoustic and pressure measurements
Low-rank Characteristic Tensor Density Estimation Part II: Compression and Latent Density Estimation
Learning generative probabilistic models is a core problem in machine
learning, which presents significant challenges due to the curse of
dimensionality. This paper proposes a joint dimensionality reduction and
non-parametric density estimation framework, using a novel estimator that can
explicitly capture the underlying distribution of appropriate reduced-dimension
representations of the input data. The idea is to jointly design a nonlinear
dimensionality reducing auto-encoder to model the training data in terms of a
parsimonious set of latent random variables, and learn a canonical low-rank
tensor model of the joint distribution of the latent variables in the Fourier
domain. The proposed latent density model is non-parametric and universal, as
opposed to the predefined prior that is assumed in variational auto-encoders.
Joint optimization of the auto-encoder and the latent density estimator is
pursued via a formulation which learns both by minimizing a combination of the
negative log-likelihood in the latent domain and the auto-encoder
reconstruction loss. We demonstrate that the proposed model achieves very
promising results on toy, tabular, and image datasets on regression tasks,
sampling, and anomaly detection
Information-theoretic Feature Selection via Tensor Decomposition and Submodularity
Feature selection by maximizing high-order mutual information between the
selected feature vector and a target variable is the gold standard in terms of
selecting the best subset of relevant features that maximizes the performance
of prediction models. However, such an approach typically requires knowledge of
the multivariate probability distribution of all features and the target, and
involves a challenging combinatorial optimization problem. Recent work has
shown that any joint Probability Mass Function (PMF) can be represented as a
naive Bayes model, via Canonical Polyadic (tensor rank) Decomposition. In this
paper, we introduce a low-rank tensor model of the joint PMF of all variables
and indirect targeting as a way of mitigating complexity and maximizing the
classification performance for a given number of features. Through low-rank
modeling of the joint PMF, it is possible to circumvent the curse of
dimensionality by learning principal components of the joint distribution. By
indirectly aiming to predict the latent variable of the naive Bayes model
instead of the original target variable, it is possible to formulate the
feature selection problem as maximization of a monotone submodular function
subject to a cardinality constraint - which can be tackled using a greedy
algorithm that comes with performance guarantees. Numerical experiments with
several standard datasets suggest that the proposed approach compares favorably
to the state-of-art for this important problem
Structural and dielectric studies of the phase behaviour of the topological ferroelectric La1-xNdxTaO4
We thank the University of St Andrews and EPSRC (via DTG studentships to CALD and JG) for funding,The layered perovskite LaTaO4 has been prepared in its polar orthorhombic polymorphic form at ambient temperature. Although no structural phase transition is observed in the temperature interval 25Ā° C < T < 500 Ā°C, a very large axial thermal contraction effect is seen, which can be ascribed to an anomalous buckling of the perovskite octahedral layer. The non-polar monoclinic polymorph can be stabilised at ambient temperature by Nd-doping. A composition La0.90Nd0.10TaO4 shows a first-order monoclinic-orthorhombic (non-polar to polar) transition in the region 250Ā° C < T < 350 Ā°C. Dielectric responses are observed at both the above structural events but, despite the ātopological ferroelectricā nature of orthorhombic LaTaO4, we have not succeeded in obtaining ferroelectric PāE hysteresis behaviour. Structural relationships in the wider family of AnBnX3n+2 layered perovskites are discussed.Publisher PDFPeer reviewe
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